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If you care to look up a chapter headed “smells, worst” in the Guiness Book of Records you will find ethyl mercaptan at the head of the list. Ethyl mercaptan is described as “rotting vegetables and sewer gas”. It can occur in wine.

I wonder if the English wine writer, Auberon Waugh, had stumbled across a bottle with a trace of ethyl mercaptan when he described a wine that his brother-in-law gave him as smelling like “dead chrysanthemums on a stillborn Jamaican baby’s grave”. Waugh was taken to court for that comment by the London Race Relations Council although the case was subsequently discharged.

I have come across a number of wines that show traces of ethyl mercaptan although I must admit that they tend to display a sort of rotten garlic or rotten onion character rather than sewer gas. Anyone who has driven past Kinleith paper mills near Tokoroa or sniffed natural gas will have experienced a whiff of mercaptan.

Mercaptan is just one of the “off odours” that can occur in wine. It is offensive when strong but in very small doses I find that it adds to the rich tapestry of smells in some wines. It can supplement a wine’s individuality and personality like a flaw in a Persian carpet. Mercaptan occurs after a wine has been bottled if that wine contained another offensive chemical, hydrogen sulphide.

Hydrogen sulphide, which is the familiar “egg yolk” smell that contaminates the atmosphere in Rotorua, is present to some degree in all wines. It often occurs during fermentation when the fermenting yeast cells run out of nutrients. When they run out of grub the yeasts understandably become stressed and start producing hydrogen sulphide, or “yeast farts” as some winemakers call it. A diligent winemaker should soon become aware that his yeasts have a touch of flatulence. The solution is simple. The winemaker will first add a yeast nutrient; such as diamonium phosphate, to the wine. That should put a smile on the yeast’s faces and stop them farting.

Because hydrogen sulphide is a gas it can sometimes be removed by sparging the fermenting wine with air. If that doesn’t work winemakers can always add copper sulphate to the wine. Copper sulphate removes the hydrogen sulphide but it also removes a bit of flavour, so it is an action of last resort.

If you discover a wine that you think may have some hydrogen sulphide there is a simple test to see whether you are right. Drop a piece of copper (an old penny works well) in the glass. If the odour diminishes you were right. I have applied this test in front of a puzzled wine waiter. He thought that I was mad but he did allow me to order another bottle from his list.

Other off-odours in wine include cork taint, a yeast infection known as brettanomyces that often smells of elastoplast, the sherry-like character of oxidation, vinegar or its associate odour of aeroplane glue, the musty odour of rotten grapes, papery smells from filter pads or the distinctive and thankfully uncommon geranium smell that can occur when a winemaker uses the preservative sorbates in red wine.

A fairly common malady in Cabernet Sauvignon-based New Zealand wines is the green and weedy odour of unripe grapes. Caused by a group of chemicals called methoxypyrazines that are often found in grapevines with an excessively dense leaf canopy.

When light strikes a leaf about 90% of that light is reflected off that leaf or absorbed by it. If the leaf is fully shading a second leaf then the shaded leaf only receives around 10% of the original light. Add a third leaf to the equation and it will receive about 1% of the original light. The result of heavy shading means a build up of methoxypyrazines and weedy wine.

Weedy wines were once very common in New Zealand. Then along came a knight in shining white armour who taught our winemakers how to reduce the level of weediness in their wines. He was a government-employed viticulturist called Dr Richard Smart. Smart by name and smart by nature. Smart showed grape growers how to thin excessively dense leaf canopies by shoot-thinning and leaf plucking. He even helped develop a machine that would mechanically trim vine leaves.

There is, according to Smart, a simple test that winemakers can use to determine whether their leaf canopies are too dense. A volunteer removes his or her clothes and stands on one side of the vine row while a second person peers through the canopy. If the leaves are too dense to determine the sex of the naked person then the canopy needs a little trimming. Don’t be concerned if during the late summer period you see naked people frolicking in the vineyard, they are simply assessing canopy density. The amount of space that you need to determine a naked person’s gender, incidentally, is about 30%.

There is a positive side to bad wine. Without bad wine we wouldn’t fully appreciate good wine. I sometimes include an indifferent (but never bad) wine in a line up of good wines to give a point of reference that will allow the tasters to see how good the good wines really are.

It is easier to identify a wine’s faults than its virtues. If you wish to impress and astound your friends offer them a glass of wine, wait until they are murmuring appreciative comments and then say “I think this wine’s got a hint of brett”. If they know what brett is they will soon begin to nod in agreement. The power of suggestion can be a weapon of mass destruction in wine tasting.

I once served friends a glass of wine from a (hidden) bottle that was sealed with a screwcap and then suggested that the wine had a suspicion of cork taint. Within a few minutes they all agreed that they could detect the characteristic mustiness of cork taint in a wine that had never been near a cork. I fooled my friends to demonstrate how we can easily trick the brain to override our sense of taste. Curiously, they were unimpressed by my performance.

Oh, and what’s the worst wine that I have ever tasted? It’s a toss up between a particularly nasty Chinese Riesling and a totally stuffed bottle of 1929 Haut Brion that tasted like something that had leaked from a Kleensak.

WINE FAULTS

Many things can go wrong in the vineyard, winery and during storage of bottled wine. Here is a list of symptoms with a brief description of the likely cause:

Symptom

Cause

Remedy (if any)

Pungent “burnt match” odour

Excess sulphur dioxide, a preservative widely used in the wine and food industry.

Aerate wine in a decanter. Hold unopened bottles for another six months before opening.

Deep orange colour (white) or brown (red), lacks fruit, bitter

Oxidation

Add a dash of cassis liqueur, tell guests that it is a fashionable French mix.

Root ginger or pyrethrum odour

Botrytis has concentrated the grape flavours before they were properly ripe. Sometimes found in sweet wines.

None.

A faint odour of glue or finger nail varnish remover.

The wine is volatile. Common in sweet wines.

Maximum permitted level 1.2 gms/litre in NZ.

Aeration in a decanter may remove some of the character.

Vinegar odour

As above

As above

Rotten egg or “Rotorua” smell

Hydrogen sulphide has formed during fermentation.

Add a copper coin or piece of copper to the wine and swirl for 20 seconds.

Rubber or burnt rubber

A “reduced”character that can result from the residue of hydrogen-sulphide when the wine is sealed with an airtight screwcap.

Aerate the wine by sloshing it into a jug or decanter and leaving to for 20-30 minutes.

About the authorBob Campbell MW

Bob Campbell MW was the second wine writer in the world (after Jancis Robinson) to become a Master of Wine. He estimates tasting over 100,000 wines since he began to formally record tasting notes.
He is regarded as "Mr New Zealand Wine" within New Zealand and around the globe. BobCampbell.nz offers access to Bob's latest wine reviews, news and opinions.